fuel cells for a sustainable energy future sossina m. haile materials science / chemical engineering...

Fuel Cellsfor a Sustainable Energy Future

Sossina M. Haile

Materials Science / Chemical EngineeringCalifornia Institute of Technology

Graduate Students: Peter Babilo, William Chueh, Lisa Cowan, Mary

Louie, Justin Ho, Wei Lai, Mikhail Kislitsyn, Kenji Sasaki, Ayako Ikeda

Former Participants: Dane Boysen, Calum Chisholm, Tetsuya Uda,

Zongping Shao, Mary Thundathil

Funding: National Science Foundation, Department of Energy, Office

of Naval Research, (past: Kirsch Foundation, Powell Foundation)

Towards a Sustainable Energy Future

Contents

• The Problem of Energy– Growing consumption– Consequences– Sustainable energy resources

• Fuel Cell Technology Overview– Principle of operation– Types of fuel cells and their characteristics

• Recent (Caltech) Advances– Too many to cover…


The Problem of Energy

• The Problem– Diminishing supply?

– Resources in unfriendly locations?

– Environmental damage?

• The Solution– Adequate domestic supply

– Environmentally benign

– Conveniently transported

– Conveniently used


World Energy Consumption

1999 totals: 400 Q-Btu, 422 EJ, 13TW2020 projections: 630 Q-Btu, 665 EJ, 21TW

Source: US Energy Information Agency(annual)264

211

158

106

52

0

Exa

Jou

les

(1018

)

8.4

6.7

5.0

3.3

1.7

0

Equ

ival

ent

Pow

er (

TW

, 10

12)

90% fossil


Fossil Fuel Supplies

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

(Exa

)J

OilRsv

OilRsc

GasRsv

GasRsc

CoalRsv

CoalRsc

Unconv

Conv

Rsv = Reserves (90%)Rsc = Resources (50%)

Source Reserves, yrs Resources, yrs Total, yrs

Oil 13 - 20 10 – 35 23 - 55

Gas 11 - 25 7 – 40 18 - 65

Coal 32 270 300

400 yrs

56-77 287-345

Source: US Energy Information Agency


US Energy Imports/Exports: 1949-2004

1957: Net Importer

• 65% of known petroleum reserves in Middle East• 3% of reserves in USA, but 25% of world consumption

Total

Coal

Petroleum

Total

Petroleum

Net

Imports Exports

Source: US Energy Information Agency

1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000

1950 1960 1970 1980 1990 2000

Qua

d B

TU

35

30

25

20

15

10

5

0

6

5

4

3

2

1

0

Qua

d B

TU

Qua

d B

TU

35

30252015105

0


Environmental Outlook

year

1000 1200 1400 1600 1800 2000

atm

osp

her

ic C

O2

[ppm

]

270

280

290

300

310

320

330

340Global CO2 levels

Source: Oak Ridge National Laboratory

2004: 378 ppm

Projections:

500-700 ppm by 2020

• Anthropogenic– Fossil fuel (75%)– Land use (25%)

Industrial Revolution


Environmental Outlook

Intergovernmental Panel on Climate Change, 2001; http://www.ipcc.chN. Oreskes, Science 306, 1686, 2004; D. A. Stainforth et al, Nature 433, 403, 2005

- 8

- 4

0

+ 4

400 300 200 100

Thousands of years before present (Ky BP)

0

T r

ela

tive t

o

pre

sent

(°C

)

300

400

500

600

700

800

CH4 (ppmv

) -- CO2

-- CH4

-- T

325

300

275

250

225

200

175

CO2 (ppmv

)

CO2 in 2004: 378 ppmv


Energy Outlook

Supply

• Fossil energy sufficient for world demand into the forseeable future

• High geopolitical risk

• Rising costs

Environmental Impact• Target

– Stabilize CO2 at 550 ppm

– By 2050

• Requires– 20 TW carbon-free power– One 1-GW power plant

daily from now until then

Urgency

• Transport of CO2 or heat into deep oceans:

– 400-1000 years; CO2 build-up is cummulative

• Must make dramatic changes within next few years


The Energy Solution

Solar1.2 x 105 TW at Earth surface

600 TW practical

Biomass5-7 TW gross

all cultivatable land not used

for food

HydroelectricGeothermal

Wind2-4 TW extractable

4.6 TW gross1.6 TW technically feasible0.9 TW economically feasible0.6 TW installed capacity

12 TW gross over landsmall fraction recoverable

Tide/OceanCurrents2 TW gross

The need:~ 20 TW by 2050

Fossil with sequestration1% / yr leakage -> lost in 100 yrs

NuclearWaste disposal


The Energy Solution

• Sufficient Domestic Supply– Coal, Nuclear, Solar

• Environmentally Sustainable Supply– Solar (Nuclear?)

• Suitable Carrier– Electricity? Hydrogen? Hydrocarbon?

• Challenges– Convert solar (nuclear) to convenient chemical form– Efficient consumption of chemical fuel


A Sustainable Energy Cycle

Solar or nuclear power plants

H2O

H2e-

Batteries

Capture

Storage

Delivery

Utilization

Hydrides? Liquid H2?

C-free Source

Hydrocarbon

???

Fuel cell

e-

H2O, CO2

+ CO2


A Few Words on Hydrogen Fuel Cells

• Hydrogen energy density– High energy content per unit mass of hydrogen

– But best storage technologies are at ~ 5 wt% H2

– 5x the weight of gasoline (for same energy content)

• Conventional hydrogen fuel cells require Pt– DOE target: 1 g/kW (0.75 g/hp)

– 100 hp engine 75g Pt $3,169

– 93% of US Pt is imported

– 80% of world reserves in one mine complex in SA• Another 15% in one mine complex in Russia

– Converting US autos would double world consumption

– 4 yr auto lifetime, 20% recycling out in 40 yrs


Fuel Cells: Part of the Solution?

• High efficiency

– low CO2 emissions

• Size independent

• Various applications

– stationary

– automotive

– portable electronics

• Controlled reactions

– “Zero Emissions”

• Operable on hydrogen

– (if suitably produced)

power plant size [MW]

0 5 10 15 20 25

eff

icie

ncy

[

%]

0

20

40

60

80automotive engine: 50-75 kW

*Can be as high as 80-90% with co-generation

*


Anode Cathode

Fuel Cell: Principle of Operation

H2 O2

H+

Overall: H2 + ½ O2 H2O

½ O2 + 2H+ + 2e- H2OH2 2H+ + 2e-

conversion device, not energy source

best of batteries, combustion engines

Electrolyte

e-


Fuel Cell Performance

H2 + ½ O2 H2O

1.17 Volts (@ no current)

• voltage losses

– fuel cross-over

– reaction kinetics

– electrolyte resistance

– slow mass diffusion

• power = I*V

• peak efficiency at low I

• peak power at mid I Current [A / cm2 ]

0.0 0.4 0.8 1.2 1.6

Vo

ltage

[V

]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Po

wer

[W /

cm2]

0.0

0.2

0.4

0.6

0.8theoretical voltagecross-over

slow reaction kinetics

electrolyteresistance

slow massdiffusion

peak power


Fuel Cell Components

• Components– Electrolyte (Membrane)

• Transport ions

• Block electrons, gases

– Electrodes

• Catalyze reactions

• Transport

– Ions, electrons, gases

• May be a composite

– (electro)Catalyst +

– Conductors +

– Pore former Membrane-Electrode Assembly (MEA)

electrolyte

catalyst

electrodes

sealant


Fuel Cell Types

Type°C[°F]

PEM90-110[200-230]

AFC100-250[212-500]

PAFC150-220[300-430]

MCFC

500-700[930-1300]

SOFC

700-1000[1300-1800]

Fuel H2 + H2O H2 H2 HC + CO HC + CO

Electrolyte

IonNafionH3O+

KOHOH-

H3PO4

H+

Na2CO3

CO32-

Y-ZrO2

O2-

Oxidant O2 O2 + H2O O2 O2 + CO2O2

By-products: H2O, CO2

Types differentiated by electrolyte, temperature of operation

Low T H2 or MeOH; High T higher hydrocarbons (HC)

Efficiency tends to as T , due to faster electrocatalysis


Fuel Cell Choices

• Ambient Temperature

Rapid start-up

H2 or CH3OH as fuels

Catalysts easily poisoned

• Applications

– Portable power

– Many on/off cycles

– Small size

• High Temperature

Fuel flexible

Very high efficiencies

Long start-up

• Applications

– Stationary power

– Auxiliary power in

portable systems

Temperature sets operational parameters & fuel choice


Technology Status

• Many, many demonstrate sites and vehicles– Stationary PAFC (200 kW) at military sites since 1995

– Stationary SOFC (100 kW) operated for 20,000 hrs

– Toyota and Honda PEM FC vehicles released 2002

– DaimlerChrysler, Ford and GM, 2005; Hyundai planned

• Legislation is a key driver– California zero emissions automotive standards

– China set for tough ‘CAFE’ standards

• Cost is a major barrier– Precious metal catalysts, fabrication, complexity

• Uncertainty in future fuel infrastructure– Gasoline for how long? Hydrogen? Methanol?


Fuel Cell System Complexity

SystemStack

Membrane electrode assembly

electrolyte

catalyst

electrodes

sealant

Electrolyte


Philosophy

Challenge

• Limitation of fuel cell materials places severe design constraints on fuel cell systems

Approach

• Material modification for improved performance

and system simplification

• New materials discovery for next generation fuel cell systems

• Novel system designs


Fuel Cell Innovations

• New Electrolytes– Intermediate temperature operation

• Lower the temperature below solid oxide fuel cells

• Raise the temperature above polymer fuel cells

• New Catalysts– Enhance reaction kinetics (improve efficiency)

– Reduce susceptibility to poisons (reduce complexity)

• Novel integrated designs– Dramatically improve thermal management

– Utilize micromachining technologies – micro fuel cells

New Electrolytes: Solid Acids

NSF & DOE Sponsored Program

(past ONR support)


SO3- + (H2O)nH+

(CF2)n

1 nm

“PEM” Fuel Cells

Nafion (Dupont)

• saturate with H2O– inverse micelle structure

• H(H2O)n+ ion transport

Proton Exchange Membrane or Polymer Electrolyte Membrane

High conductivity Flexible, high strength

Requires humidification & water management

Operation below 90°C Permeable to methanol

Target: 120-300°C; examine inorganic H+ conductors

H(H2O)n+

H2O

Kreuer, J Membr Sci 2 (2001) 185.


Solid Acids

• Chemical intermediates between normal salts and normal acids: “acid salts”½(Cs2SO4) + ½(H2SO4) CsHSO4

• Physically similar to salts

• Structural disorder at ‘warm’ temperatures

• Properties

disordereds truc ture

norm al s truc ture

s truc tura ltrans ition

1/TT

log

(co

nd

uct

ivity

)

polym er

Direct H+ transport

Humidity insensitive

Impermeable

Water soluble!! Brittle


Proton Transport Mechanism

Sulfate groupreorientation

10-11 seconds

Proton transfer

10-9 seconds

H

O

S


1000/T K-1

2.00 2.25 2.50 2.75 3.00 3.25

log

(con

duct

ivity

) [S

/ c

m]

-8

-7

-6

-5

-4

-3

-2

-125°C50°C100°C150°C250°C

Conductivity of Solid Acids

• CsHSO4 [Baranov, 1982]

• CsHSeO4 [Baranov, 1982]

• (NH4)3H(SO4)2 [Ramasastry, 1981]

• Rb3H(SeO4)2 [Pawlowski, 1988]

• Cs2(HSO4)(H2PO4)[Chisholm & Haile,

2000]

• -Cs3(HSO4)2(H2PO4)[Haile et al., 1997]

• K3H(SO4)2 [Chisholm & Haile,

2001]

Superprotonic transition

But sulfates and selenates are unstable under reducing conditions…


Solid Acid Properties

The Good

• H+ transport only

– No electro-osmotic drag

– No electron transport

– Alcohol impermeable

• Humidity insensitive

conductivity

• Stable to ~ 250ºC

• Inexpensive

• Chemically non-aggressive

The Bad

• Known compounds are

water soluble

– Operate at T 100ºC

– Insoluble analogs?

• Few are chemically stable

The Ugly

• Poor processability and

mechanical properties

– Composite membranes

with inert polymers


CsH2PO4 as a Fuel Cell Electrolyte

• Expected to have chemical stability

– 3CsH2PO4 + 11H2 Cs3PO4 + 3H3P + 8H2O

– dG(rxn) >> 0

• But does it have high conductivity?

• Does it have sufficient thermal stability?

• Literature dispute

– High conductivity on heating due to H2O loss

– High conductivity due to transition to a cubic phase


200 225 250 275 3000.0

0.1

0.2

0.3

0.4

47

62

71

77

Equ

ilibr

ium

pH

2O(a

tm)

Temperature(C)

Wat

er te

mpe

ratu

re (

o C)

CsH2PO4 Dehydration

CsH2PO4 CsPO3 + H2O

Tc = 230 Toper = 250°C

P(H2O) operation

Use water partial pressure to suppress dehydration

CsH

2-2x

PO 4-

x

CsH 2

PO4

pressure detector

CsH2PO4 + H2O


260 240 220 200 180 160

1.9 2.0 2.1 2.2 2.3 2.4

-6

-4

-2

log

(co

nd

uct

ivity

)

[-1cm

-1]

1000/T [K-1]

1st heating 1st cooling 2nd heating 2nd cooling

Temperature [oC]

Conductivity of CsH2PO4

230 °C

0.42 eV

Humidified airp[H2O] = 0.4 atm


0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

0

10

20

30

40

50

60

Cel

l vol

tage

(V

)

Current density (mA cm-2)

power

voltage

before after

Pow

er d

ensi

ty (

mW

cm

-2)

Proof of Principle

Compared to polymers High open circuit voltage

– Theoretical: 1.15V

– Measured: 1.00 V

– Polymers: 0.8-0.9 V

Power density– Polymers: > 1 W/cm2

• Platinum content– Polymers: ~ 0.1 mg/cm2

H2O, H2 | Cell | O2, H2O

260m membrane; 18 mg Pt/cm2

T = 235ºC 50 mW/cm2

D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M. Haile, Science 303, 68-70 (2004)


0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

offon, 100 mA/cm2

Cel

l vol

tage

(V

)

Time (h)

Fuel Cell Longevity: Stable Performance

H2, H2O | cell | O2, H2O

260 m thick CsH2PO4 electrolyteT = 235ºCCurrent = 100 mA/cm2

• CsH2PO4 – no degradation in 110 hr measurement

• CsHSO4 – functions for only ~ 30 mins (recoverable degradation)


3rd Generation Fuel Cell

T. Uda & S.M. Haile, Electrochem & Solid State Lett. 8 (2005) A245-A246

H2, H2O | cell | O2, H2O

T = 248°C8 mg Pt/cm2

10-40 m pores, ~40% porosity

Slurry deposit

Fine CsH2PO4

2 m

Open circuit voltage: 0.9-1.0 V Peak power density: 285-415 mW/cm2


Impact

Some Like It Medium Hot

The promise of protonics Solid Acids Show Promise...

Nature: News & ViewsScience Now Magazine

Physics Today Online

S. M. Haile, D. A. Boysen, C. R. I. Chisholm and R. B. Merle, “Solid Acids as Fuel Cell Electrolytes,” Nature 410, 910-913 (2001).


Fuel Cell Stack


‘Direct’ Alcohol Fuel Cells

• SAFCS ideal thermal match– Reforming rxn: 200 – 300°C– Electrolyte: 240 – 280°C– Steam reforming: endothermic– Fuel cell rxns: exothermic

• Integrated design– Incorporate alcohol reforming

catalyst in anode chamber

Methanol in proton exchange membrane fuel cellsCH3OH + H2O 6H+ + CO2 + 6e-

CH3OH + H2O 3H2 + CO2


‘Direct’ Methanol Fuel Cells

0.0

0.1

0.2

0.3

0.4

0.0 0.5 1.0 1.50.0

0.2

0.4

0.6

0.8

1.0

Ce

ll vo

ltag

e (

V)

0.0 0.5 1.0 1.5

Current density (A/cm2)

Po

we

r d

en

sity

(W

/cm

2)

With reformer Without reformer

hydrogen

methanol42 vol%

hydrogen

methanol

reformate1%CO

24%CO2

• Methanol power ~ 85% H2 power

• For polymer fuel cells ~ 10%

• Reformate power ~ 90% H2 power

• Methanol power ~ 45% H2 power

T = 260 °C; 47 m membrane T = 240 °C; 34 m membrane

1.5


‘Direct’ Alcohol Fuel Cells

T = 260 °C; 47 m membrane

With reformer

ethanol36 vol%

Vodka80 proof

• Ethanol power ~ 40% H2 power

New Cathodesfor Solid Oxide Fuel Cells

NSF & DOE Sponsored Program

(past DARPA support)


State-of-the-Art SOFCs

• Operation: ~1000 °C• Fuel flexible, efficient• But…

– All high temp materials– Costly (manufacture)– Poor thermal cyclability

• Goal: 500 – 800 °C

• Challenges– Slower kinetics – Electrolyte resistance– Poor anode activity– Poor cathode activity

cathode (air electrode)

anode (fuel electrode)

Component Materials

(La,Sr)MnO3

Zr0.92Y0.08O2.96 = yttria stabilized zirconia (YSZ)

Ni + YSZ composite

electrolyte


Solid Oxide Fuel Cell Cathodes

• Traditional cathodes– A3+B3+O3 perovskites

– Poor O2- transport– Limited reaction sites

• Our approach– High O2- flux materials– Extended reaction sites

– A2+B4+O3 perovskites

electrolyteO2-

O2

Oad

2e-cathode

Oad

electrolyteO2-

O2Oad

2e- cathodeO2-

‘triple-point’ path electrode bulk path

(Ba0.5Sr0.5)(Co0.8Fe0.2)O2.3

almost 1 in 4 vacant


Cathode Electrocatalysis

• Symmetric cell resistance measurements

Ag current collectors

ElectrolyteO2, Ar,

(CO2)

cathode layers

+

-

½ O2

2e-

O=

½ O2

2e-

+

-

Rcathode

Rcathode

Relectrolyte

• Equivalent circuit– Distinguish resistance

contributions using frequency dependent measurements


1.0 1.1 1.2 1.3 1.4 1.5

0.01

0.1

1

10750 700 650 600 550 500 450 400

0.01

0.1

1

10

Temperature (oC)

Cat

hode

are

a sp

ecifi

c re

sist

ance

(.

cm2 )

1000/T (K-1

Symmetric cell (2-electrode) Half cell (3-electrode)

Ea=116 kJ.mol-1

Cathode Electrocatalysis

Ea same as oxygen surface

exchange (113 kJ.mol-1)

Bulk diffusion is fast (46 kJ.mol-1)

Other ‘advanced’ cathodes

(PrSm)CoO3: 5.5 cm2

(LaSr)(CoFe)O3: 48 cm2

0.5 – 0.6 cm2

P(O2) = 0.21 atm electrolyteO2-

O2 Oad

cathodeO2-

slowfast

2e-


Cell Fabrication

Spray cathode

NiO + SDC (Ce0.85Sm0.15O2)

NiO + SDC

SDC

Dual dry press

Sinter, 1350oC 5h

600oC 5h, 15%H2

Porous anode

Calcine, 950oC 5h, inert gascathode

electrolyte

anode

Anode supported

Anode: 700 m

Cathode: 20 m

Electrolyte

~ 20m

Electrolyte surface

2 m

1.3

cm

0.71 cm2


0 1000 2000 3000 40000.0

0.2

0.4

0.6

0.8

1.0

0

200

400

600

800

1000

Pow

er d

ensi

ty (

mW

.cm

-2)

Vol

tage

(V

olts

)

Current density (mA.cm-2)

600oC

550oC

500oC

445oC

400oC

Fuel Cell Power Output

> 1 W/cm2 at 600°C!!!H2, 3% H2O | fuel cell | Air

Comparison: literature cathode material 350 mW/cm2 at 600°C


Impact

Cooler Material Boosts Fuel Cells

SOFC cathode is hot stuff…Next generation of fuel cells…

Tech Research News

R & D FocusFuel Cell Works

Z. Shao and S. M. Haile, “A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells,” Nature 431, 170-173 (2004).


Summary & Conclusions

• Sustainable energy is the ‘grand challenge’ of the 21st century– Solutions must meet the need, not the hype

– Fuel cells can play an important role

• Solid acid fuel cells– Radical alternatives to state-of-the-art

– Viability demonstrated; spin-off company established

• Solid oxide fuel cells– Promising alternative cathode discovered

• Still plenty of need for fundamental research

“The stone age didn’t end because we ran out of stones.”-Anonymous


Acknowledgments

• The people

• The agencies– National Science Foundation, Office of Naval Research,

DARPA, California Energy Commission, Department of Energy, Kirsch Foundation, Powell Foundation

Calum Dane

TetsuyaMary

Kenji

Zongping

Justin

Wei


Selected Relevant Publications

• T. Uda, D. A. Boysen, C. R. I. Chisholm and S. M. Haile, “Alcohol Fuel Cells at Optimal Temperatures,” Electrochem. Solid State Lett. 9, A261-A264 (2006).

• T. Uda and S. M. Haile, “Thin-Membrane Solid-Acid Fuel Cell,” Electrochem. Solid State Lett. 8, A245-A246 (2005).

• D. A. Boysen, T. Uda, C. R.I. Chisholm and S. M. Haile, “High performance Solid Acid Fuel Cells through humidity stabilization,” Science Online Express, Nov 20, 2003; Science 303, 68-70 (2004).

• D. A. Boysen, S. M. Haile, H. Lui and R. A. Secco “High-temperature Behavior of CsH2PO4 under both Ambient and High Pressure Conditions,” Chem. Mat. 15, 727-736 (2003).

• R. B. Merle, C. R. I. Chisholm, D. A. Boysen and S. M. Haile, “Instability of Sulfate and Selenate Solid Acids in Fuel Cell Environments,” Energy and Fuels 17, 210-215 (2003).

• S. M. Haile, D. A. Boysen, C. R. I. Chisholm and R. B. Merle, “Solid Acids as Fuel Cell Electrolytes,” Nature 410, 910-913 (2001).

• Z. P. Shao, S. M. Haile, J. M. Ahn, P.D. Ronney, Z. L. Zhan and S. A. Barnett, “A thermally self-sustained micro Solid-Oxide Fuel Cell stack with high power density,” Nature 435, 795-798 (2005).

• Z. Shao and S. M. Haile, “A High Performance Cathode for the Next Generation Solid-Oxide Fuel Cells,” Nature 431, 170-173 (2004).

• S. M. Haile, “Fuel Cell Materials and Components,” (invited) Acta. Met. 51, 5981-6000 (2003).

• S. M. Haile, “Materials for Fuel Cells,” (invited) Materials Today 18, 24-29 (2003).

fuel cells for a sustainable energy future sossina m. haile materials science / chemical engineering...

Documents

department of energy

fossil slide

ppmv slide

powell foundation slide

source reserves

tw source

industrial revolution

anthropogenic fossil